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While lazy students may need a kick in the butt, certain mechanisms of brain dysfunction seem to spring into action with a kick in the gut. So suggest two recent papers showing how peripheral inflammation can intensify central nervous system (CNS) damage. In the 22 October Journal of Neuroscience, Malu Tansey and colleagues at the University of Texas Southwestern Medical Center, Dallas, report that prolonged intraperitoneal administration of low-dose lipopolysaccharide (LPS) in a Parkinson’s mouse model triggered subtle motor defects and neurodegeneration that do not develop with parkin gene loss or LPS alone. In a paper published in this week’s PNAS Early Edition, researchers in Canada led by Quentin Pittman at the University of Calgary, Alberta, have found that short-term gut inflammation in rats can boost brain excitability and susceptibility to seizures.

About 4 percent of Parkinson disease patients develop symptoms before age 50 (Van Den Eeden et al., 2003), with roughly half these early onset cases from loss-of-function mutations in the gene for parkin (Lücking et al., 2000), an E3 ubiquitin ligase. Among the telltale signs of PD is significant loss of dopamine-producing neurons in the substantia nigra, a brain region important for controlled movement. Parkin knockout mice, however, display no such nigral degeneration (see, e.g., Goldberg et al., 2003 and Zhu et al., 2007). “Our hypothesis was that what these mice were missing was an environmental trigger that would potentially cause them to lose neurons,” said lead author Tamy Frank-Cannon, a former postdoc in Tansey’s lab who is now at Texas A&M University in College Station. Involvement of an environmental “second hit” seems plausible in people as well, Tansey noted in an e-mail to ARF. Though two mutant parkin alleles pretty much guarantee PD, the age of onset can vary considerably (up to 20 years) among family members with the same mutation (Lücking et al., 2000).

Based on extensive work implicating abnormal neuron-glia interactions in neuronal loss, and human prospective studies linking inflammatory processes to increased PD risk, Tansey’s team figured that parkin-deficient mice might need an inflammatory stimulus to spur loss of nigral dopaminergic neurons. The researchers injected wild-type and parkin-/- mice intraperitoneally with low-dose LPS, or saline control, twice a week for three or six months. A separate group of animals received these injections for three months, followed by a three-month lag. Behavioral assessments were taken at baseline and every three months. As proof of concept, two months of the twice-weekly injections could trigger a CNS inflammatory response—measured as mRNA increases in Cox-1 and CD45 in the olfactory bulb and midbrain, respectively, and not in brain areas unaffected in early PD.

Frank-Cannon and colleagues found no gross motor differences (assessed by open-field and rotarod performance) in any of the treatment groups. However, parkin knockout mice that got LPS injections for three or six months straight did significantly worse than saline-treated or wild-type controls on a beam-walk test for fine motor skills.

After running the behavioral tests, the researchers killed the animals and looked, in immunostained midbrain sections, for loss of dopamine-producing neurons. They found it, using an antibody to the dopaminergic neuron marker TH, in the substantia nigra pars compacta (SNpc) of parkin-null mice that got either six months of LPS treatment or three months of treatment followed by a three-month lag. The neurodegeneration appeared selective to nigral neurons, as numbers of TH-positive neurons in the neighboring ventral tegmental area (VTA) remained unchanged in LPS-treated parkin-/- mice relative to wild-type and saline-injected controls.

“The fact that nigral neuron loss was not accompanied by striatal pathology, such as fiber degeneration and dopamine depletion, appears puzzling at first glance,” wrote Konstanze Winklhofer of Adolf-Butenandt-Institute in Munich, Germany, in an e-mail to ARF. “On the other hand, this observation might indicate that inflammatory responses are regulated differently in the substantia nigra and striatum, explaining the high vulnerability of nigral neurons to inflammatory stimuli, such as TNF.” (See full comment below.) Tansey noted that ongoing studies in her lab are testing possible mechanisms by which parkin may regulate the functional outcome of inflammatory stress.

Providing another example of interplay between peripheral inflammation and CNS dysfunction, Pittman’s team looked at how short-term inflammatory activity in the gut affects neuronal excitability in adult rats. First author Kiarash Riazi and colleagues injected the animals with TNBS (2,4,6-trinitrobenzene sulfonic acid), triggering a T helper-1 cell-mediated model of inflammatory bowel disease that causes localized colitis. In TNBS-treated animals, the researchers detected maximal increased seizure susceptibility in response to the convulsant pentylenetetrazole (PTZ) on day 4, corresponding to peak inflammation severity. Using milder inflammatory regimens, they found a strong correlation between the extent of bowel damage and seizure susceptibility scores. To rule out the possibility that these effects could be chalked up to side effects of TNBS treatment that enabled quicker brain penetration or slower metabolism of PTZ, the researchers showed that hippocampal brain slices from TNBS-treated rats were intrinsically more excitable in an in-vitro model of epileptogenesis.

In immunostained brain sections, Pittman and colleagues found greater proportions of activated microglia in the dentate gyrus, entorhinal cortex, and CA1 and CA3 areas of rats that had received TNBS four days earlier, compared to saline-treated controls. These differences faded by 10 days post-treatment. To show that activated microglia indeed contributed to the higher CNS excitability, the researchers blocked microglial activation with daily minocycline injections into the brain. Four days of minocycline treatment brought seizure susceptibility scores in TNBS-treated rats down to those of saline-injected controls. Furthermore, at day 4, levels of the proinflammatory cytokine TNFα were significantly higher in TNBS-treated rats than in control animals. Once again, blocking TNFα with a neutralizing antibody prevented the TNBS-induced increase in seizure susceptibility. What’s more, chronic TNFα microinfusions into the brains of non-inflamed rats seemed to increase seizure susceptibility scores about as much as did the colon treatment, suggesting a role for TNFα in the inflammation-induced CNS effects.

“When there’s chronic peripheral disease, there are going to be cytokines released in the brain, and those cytokines can have a number of effects that could help to explain some types of comorbidity,” Pittman told ARF. Though the new data are perhaps most relevant to comorbidities shared by peripheral inflammatory disease and epilepsy, mechanisms of inducing sub-threshold seizures in the hippocampus are of great interest to Alzheimer disease researchers in light of a recent study that uncovered such seizures in an amyloidogenic AD mouse model (see ARF related news story).

Pittman believes the colitis-induced CNS effects extend well beyond seizure susceptibility. “These animals with colitis also show very different abilities of neurons in cellular models of learning,” he said. He and colleagues have started looking at how bowel inflammation spurs changes in synaptic strength and long-term potentiation, for example.

Whether it be gut injections of LPS or TNBS, “both had major impact on what's going on in brain,” Frank-Cannon said of the two studies. “The fact that peripheral inflammation can actually do that is one of the new and emerging themes in a lot of neurodegenerative diseases right now that I think we're just starting to recognize.”—Esther Landhuis

Comments on News and Primary Papers

The etiopathogenesis of sporadic Parkinson disease (PD) is only poorly understood. Over the past decade, several genes have been linked to monogenic familial variants of PD, which can provide insight into the pathomechanisms and allow the generation of PD-specific animal models. Mutations in the parkin gene (PARK2), encoding a E3 ubiquitin ligase, are responsible for the majority of autosomal recessive parkinsonism. Parkin knockout mice show some minor alterations in dopaminergic neurotransmission; however, they do not display overt degeneration of the nigrostriatal pathway. The authors of this study therefore reasoned that an additional "hit" is needed to induce loss of dopaminergic neurons in parkin knockout mice, consistent with the idea that genetic and environmental factors play a role in the pathogenesis of PD.

Using a paradigm of chronic inflammation triggered by serial low-dose intraperitoneal LPS injections, the authors first show that this regimen induces a specific neuroinflammatory response in brain regions which are affected in early stages of PD: the midbrain and olfactory bulb. In contrast to wild-type mice, parkin knockout mice developed fine-locomotor deficits in response to the inflammatory stimulus. Remarkably, systemic LPS treatment increased the vulnerability to nigral dopaminergic neuron loss in parkin knockout mice, while the profile and extent of the midbrain neuroinflammatory response in wild-type and parkin knockout mice was similar.

The fact that nigral neuron loss was not accompanied by striatal pathology, such as fiber degeneration and dopamine depletion, appears puzzling at first glance. On the other hand, this observation might indicate that inflammatory responses are regulated differently in the substantia nigra and striatum, explaining the high vulnerability of nigral neurons to inflammatory stimuli, such as TNF. In line with the current study is the recent finding that parkin has a permissive effect on NFκB signaling. It will now be interesting to address the question of how neuroinflammation and nigral degeneration are linked mechanistically and whether neuroinflammation plays a pathogenic role specifically in patients with pathogenic parkin mutations.